The present invention relates in general to lasers (ultrafast lasers) which deliver radiation as a sequence of ultrashort pulses having a duration from a few picoseconds (ps) to as short as a few femtoseconds (fs). The invention relates in particular to automatic tuning of modelocked ultrafast lasers while minimizing changes in pulse characteristics thereof.
Modelocked ultrafast lasers employing a solid state-gain-medium such as Ti:sapphire can be tuned over a relatively wide range of wavelengths. By way of example, a Ti:sapphire laser can be tuned from about 700 nanometers (nm) to about 1000 nm. A preferred modelocking method for such a laser is Kerr lens modelocking (KLM).
The broad tuning range and the modelocked, ultrashort pulses delivered by such lasers find application in several branches of science including materials processing, spectroscopy, medicine and biology. There are several technical problems, however, which complicate tuning an ultrafast laser over the full wavelength range characteristic of the laser gain-medium.
One problem is that the laser gain varies over the full tuning range of the laser. For a KLM, Ti:sapphire laser, intracavity power has to be held with a certain range to maintain stable, modelocked operation. Because of this, as the laser is tuned, pump-power to the gain-medium must be adjusted to compensate for the variation in gain. Tuning is often accomplished by providing an adjustable wavelength selective device, such as a birefringent filter (BRF), in the laser cavity. Pump power must be lowered or raised as gain respectively increases or decreases in response to a tuning adjustment.
A second problem is that in an ultrafast laser some compensation must be provided for group-delay-dispersion compensation of spectral (wavelength) components of a pulse. Such compensation is required to prevent broadening of pulses due to differing circulation times in the laser resonator for different wavelength components of the pulse. In a broadly tunable laser, group-delay-dispersion compensation is usually provided by one or more prism pairs. These prisms cause the different wavelength components of a pulse to have different optical path lengths in the laser cavity.
As the nominal wavelength (center wavelength of the wavelength components of the pulse) of the laser is tuned, however, the intracavity laser beam dispersed by a first prism of a pair is incident on the second prism of the pair at different positions and follows a different path through the prism. This difference can be sufficient that group-delay-dispersion compensation characteristics of the prism pair at different tuning (center) wavelengths are sufficiently different that pulse characteristics of the laser, such as temporal shape, bandwidth, and duration, as well as power, can change significantly as the laser is tuned. In such cases, rearrangement of the prisms is necessary to maintain the same pulse characteristics from one tuning wavelength to the next. Consistency of such characteristics is advantageous in minimizing variables in applications of the laser. A third problem related to group-delay-dispersion compensating prisms is that as the intracavity beam path changes, the effective alignment of a resonator with respect to that beam path can also change, resulting in a change of power in the intracavity beam.
A result of these problems is that for many experiments wherein a tunable ultrafast laser is used, experimental progress is inhibited by a need for frequent readjustment of laser components to compensate for changes in laser output pulse characteristics resulting from tuning. Because of the complex nature and inter-relationship of the adjustments, such adjustments are best made by persons skilled in the art. Experimenters using the lasers often may not have such laser adjustment skills. It is believed that these problems provide a significant obstacle to applications of ultrashort pulse technology where broad wavelength scanning is required. It is believed that this obstacle can best be overcome by providing a tunable ultrafast laser wherein such adjustments could be made automatically.
In prior-art ultrafast laser configurations, providing an automated tuning arrangement that could automatically adjust a laser in response to a single tuning command would, at best, be extremely difficult, again due to the complex and transcendental inter-relationship of variables resulting from tuning adjustments. Even given sensors for measuring pulse wavelength, bandwidth and duration characteristics as well as laser output power, arriving at a reliable control algorithm to automatically make the required adjustments would be a formidable task. There is a need for an ultrafast laser arrangement which requires sufficiently few tuning adjustments that automatic operation can be simplified with automatic control preferably based on monitoring a single output parameter of the laser.
The present invention is directed to a laser comprising a laser resonator having a gain-medium located therein. The gain-medium, when energized, provides optical gain over a band of wavelengths characteristic of the gain-medium. The band of wavelengths is defined as a gain-bandwidth.
An optical pump is arranged to energize the gain-medium, thereby causing laser radiation to circulate within the laser resonator. An optical switch is located in the laser resonator and arranged such that the laser radiation circulates as sequence of pulses characterized by a center wavelength and a pulse-bandwidth.
A plurality of prisms is located in the laser resonator and arranged for providing group-delay dispersion compensation. At least one of the prisms is moveable such that the prisms have a selectively variable spatial relationship with each other. The spatial relationship is arranged such that pulses of different wavelengths within the gain-bandwidth, in passing through the prisms, are caused to follow different optical paths in the resonator.
A stop is configured and positioned in the resonator cooperative with the prisms to limit the different optical paths possible in the resonator such that only those of the pulses having a selected center wavelength can circulate in the resonator. The spatial relationship of the stop with the prisms is selected such that the circulating pulses follow a predetermined optical path-length therein.
An arrangement is provided for moving the at least one and any other movable prisms cooperative with the stop for varying the selected center wavelength of the pulses within the gain-bandwidth. The cooperative movement is arranged such that the predetermined optical path-length of the pulses in the prisms remains about the same as different pulse center wavelengths are selected by the prism motion.
In one aspect of the inventive laser, the optical switch is arranged for passive modelocking at a predetermined range of power of the circulating radiation. The laser includes an electronic controller arranged to control the cooperative movement arrangement. The controller is further arranged to vary power delivered by the optical pump to the gain-medium such that the circulating laser power remains within the predetermined range thereof as different center wavelengths of the pulses are selected.
In one preferred embodiment of the inventive laser there are two prisms. A first of the prisms is moveable with respect to the second and the stop is maintained in a fixed relationship with the first prism, the relationship being such that when the first prism is moved the stop is synchronously moved therewith.